1. Field of the Invention
The present invention relates to data processing systems and, in particular, to circuitry external to a data processing system which utilizes the contents of to map to a new value which is derived from the contents of the register.
2. Discussion of the Prior Art
In a data processing system, there are a number of functions that must be performed on each data character that is received or transmitted by the system. Examples of these mandatory functions include: switching contacts between the normal or "user" state and the state where character interrupts are processed; computing error detection polynomials; translating characters from one code set to another; and testing for normal or abnormal termination of a data stream. It is this required processing that limits the rate at which a computer operates. If the character bandwith of a data communications system is to be maximized, then these functions must be expedited.
In the early development of data processing systems, if high performance communication was required, it was necessary to develop custom hardware for the communications protocol to be implemented. For example, IBM used special, single-protocol hardware for its early Bisync and PARS/IPARS implementations.
In the 1950's, data processing systems that could service "character at a time" interrupts from a single communication line operating at the rate of 2000 bits per second were extremely expensive, specialized systems which were economically beyond the reach of most commercial organizations. With the development of microprocessor integrated circuits, the cost of data communication systems has become more reasonable and their use widespread.
However, as data communications hardware becomes more commonplace, users become more imaginative in its application. The present demand is for a system that permits operation of any communications protocol at data rates of 64000 bits per second over digital communications channels. Using a 6-bit protocol, such as is commonly used in the airline industry, for example, this corresponds to a data rate of 10667 characters per second in each direction, or a perbandwidth data rate of over 21000 characters per second. To support these rates, either custom hardware or expensive "bit-slice" microprocessors would typically be necessary. Unfortunately, these techniques do not provide general solutions and, therefore, force the expenditure of much more development effort for each new protocol than should be required.
Therefore, it would be extremely advantageous if an inexpensive, general purpose solution could be provided to implement the mandatory processing functions associated with differing data communication protocols.